Marine trunk piston engines generally use Heavy Fuel Oil (‘HFO’) for offshore running. Heavy Fuel Oil is the heaviest fraction of petroleum distillate and comprises a complex mixture of molecules including up to 15% of asphaltenes, defined as the fraction of petroleum distillate that is insoluble in an excess of aliphatic hydrocarbon (e.g. heptane) but which is soluble in aromatic solvents (e.g. toluene) as measured by ASTM D6560. Asphaltenes can enter the engine lubricant as contaminants either via the cylinder or the fuel pumps and injectors, and asphaltene precipitation can then occur, manifested in ‘black paint’ or ‘black sludge’ in the engine. The presence of such carbonaceous deposits on a piston surface can act as an insulating layer which can result in the formation of cracks that then propagate through the piston. If a crack travels through the piston, hot combustion gases can enter the crankcase, possibly resulting in a crankcase explosion.
It is therefore highly desirable that trunk piston engine oils (‘TPEO’s) prevent or inhibit asphaltene precipitation, a problem which becomes more acute when the oil of lubricating viscosity has a higher saturates content. The prior art describes ways of doing this by use of metal carboxylate detergents in combination with a polyalkenyl-substituted carboxylic acid anhydride. WO 2010/115594 ('594) and WO 2010/115595 ('595) describe the use, in trunk piston marine engine (TPEO) lubricating oil compositions that contain 50 mass % or more of a Group II basestock, of respective minor amounts of a calcium salicylate detergent and of a polyalkenyl-substituted carboxylic acid anhydride. The data therein shows that the combination gives rise to improved asphaltene dispersency. EP-A-2644687 ('687) describes the use of a combination of defined calcium salicylates and defined polyalkenyl-substituted carboxylic acid anhydrides in a TPEO lubricant comprising a major amount of an oil of lubricating viscosity containing 50 mass % or more of a Group I basestock. This achieves good asphaltene dispersency at lower, and hence more economical, levels of soap.
The art does not, however, concern itself with the influence of the succination ratio of the anhydride in such combinations on the problem of asphaltene precipitation such as at higher saturate levels in the oil of lubricating viscosity in a TPEO. Component (B) in the examples of '594 is stated to be a PIBSA derived from a polyisobutene of number average molecular weight 950; its succination ratio is not stated.